Lattice mismatch is a key parameter for semiconductor growth techniques. Typically, growth of high quality epitaxial material on a lattice matched substrate is significantly easier than growth of comparable quality material on a lattice mismatched substrate. In particular, growth on a lattice mismatched substrate tends to lead to the formation of threading dislocations (and other defects, such as rough surfaces) which extend upward from the substrate and pass through active regions of semiconductor devices, thereby degrading device performance and/or reliability.
Improving the quality of lattice mismatched growth by reducing defect density has been extensively investigated in the art. Various techniques have been described. For example, U.S. Pat. No. 6,184,144 considers various growth methods which are intended to provide inherently dislocation-free growth. More commonly, lattice mismatched growth is accomplished by growing a buffer structure on the substrate, then growing a device structure on top of the buffer structure. The top surface of the buffer structure is lattice matched to the device structure, so growth of the device structure on the buffer is not lattice mismatched, even though the device structure is lattice mismatched relative to the substrate.
Several different buffer structures and buffer growth methods have been described in the art, as applied to various material systems. For example, U.S. Pat. No. 5,107,317 considers growth of GaAs or a GaAs containing alloy on a Si substrate with a two-layer buffer structure. The first buffer layer is GaAs or a GaAs containing alloy, and the second buffer layer is Ge or a Ge containing alloy. U.S. Pat. No. 6,987,310 considers growth of SiGe on Si where a three-layer buffer structure is grown, then annealed (e.g., 1 hour at 950° C.). U.S. Pat. No. 5,659,187 considers a compositionally graded buffer. U.S. Pat. No. 6,724,008 considers a compositionally graded buffer for growth of SiGe on Si, where the buffer is planarized prior to device growth to alleviate the surface non-uniformity typical of thick graded SiGe buffers.
In the preceding examples of compositional grading, the grading is continuous. Compositional grading can also be performed in discrete increments. For example, U.S. Pat. No. 6,864,115 considers a buffer structure for growth of SiGe on Si having multiple SiGe layers, each layer having a uniform composition. Grading is implemented by gradually increasing the Ge concentration in successive buffer layers deposited on a Si substrate. Growth is carried out at relatively low temperatures, and a higher temperature anneal is employed after growth of each buffer layer (or each few buffer layers) to reduce defect density.
Although compositional grading (discrete or continuous) has the advantages of typically providing the lowest available defect densities, and being generally applicable to a wide variety of material systems, it also has significant disadvantages. In particular, compositionally graded buffers tend to be thick, which increases growth cost, and can have other undesirable practical consequences (e.g., increased surface non-uniformity, mechanical fragility, CTE mismatch). This has motivated the development of non-graded buffer approaches (e.g., U.S. Pat. No. 5,107,317 and U.S. Pat. No. 6,987,310 above). Although non-graded buffer approaches advantageously reduce buffer thickness, these approaches tend not to have the general applicability of graded buffers. Instead, non-graded buffer approaches rely on specific structures and/or methods to reduce defect density. Often these structures and methods are also specific to a particular material system, and are not readily applicable to significantly different material systems.
For the SiGe material system, graded buffers have been investigated (e.g., U.S. Pat. No. 6,724,008, U.S. Pat. No. 6,864,115). Non-graded buffers in the SiGe system have also been investigated (e.g., the three layer buffer of U.S. Pat. No. 6,987,310). Since this three layer buffer structure is relatively complicated, it would be an advance in the art to provide simpler lattice mismatched SiGe growth.